Field of the Invention
The invention relates to a method for making electrical contact with the rear side of a semiconductor substrate when processing the semiconductor substrate.
When manufacturing semiconductor components, process steps are used which require an electrical contact to the substrate to be processed. This is the case, for example, in electrical or electrochemical process steps. In order to be able to manufacture a large number of identical components in parallel on a semiconductor substrate, it is necessary for the substrate to be processed uniformly by the process step. This requires contacting methods which make electrical contact with the substrate in a way which is as homogenous as possible. If a homogenous electrical contact is not ensured, there is a variation in the electrical potential over the substrate, which can become manifest in an inhomogenous process control and can prevent a uniform execution of the process step. The fluctuation leads to a non-uniform galvanic deposition (in the case of a negative substrate potential) and to non-uniform anodic dissolving (in the case of a positive substrate potential).
In the case of the anodic dissolving of the substrate, given, for example, a suitably selected doping and electrolyte composition, pores are formed when there is a low anodic potential and electropolished surfaces are formed when there is a high anodic potential. This constitutes a considerable dependence of the semiconductor components to be formed on the potential applied and may prevent that properly functioning semiconductor components are formed.
The formation of pores in silicon is of interest, for example, for manufacturing trench capacitors because the formation of pores allows a considerable increase of the surface and thus allows to realize an associated increase in capacitance. Especially suitable pores are referred to as xe2x80x9cmesoporesxe2x80x9d with a pore diameter in the range of 2-10 nanometers (nm). Since, as already mentioned above, the formation of pores depends on the electrical potential, it is of great importance to apply this potential as uniformly as possible over the substrate.
Published, Non-Prosecuted German Patent Application DE 197 28 962 A1 describes, for example, a device for etching a semiconductor wafer. The device is suitable, for example, for etching a main surface of the semiconductor wafer. Electrical contact is made with the semiconductor wafer from the rear side in its edge region.
Japanese Patent Document JP 59-41830 A relates to a mounting device for a semiconductor substrate, through the use of which a fluid for electroplating the semiconductor substrate is sprayed onto the semiconductor substrate. Here also, the electrical contact is made in the rear-side edge region.
U.S. Pat. No. 4,428,815 relates to a vacuum mounting device for mounting fragile objects such as, for example, semiconductor substrates. Electrical contact is made through the use of rear-side contact elements in the center of the substrate.
U.S. Pat. No. 5,437,777 relates to a system for depositing a wiring plane on a semiconductor wafer. Needle electrodes are installed on the front side of the substrate in order to make electrical contact with the front side of the substrate.
Japanese Patent Document JP 10-046394 A relates to an electroplating system and a method for a galvanic depositing, for example, of a metallic layer. Here, a mounting device for the semiconductor wafer is embodied in such a way that the contact is made with the semiconductor wafer at its periphery.
Japanese Patent Document JP 62-293632 A describes an insulating layer which is provided with holes and is provided between the semiconductor wafer and a chuck.
Japanese Patent Document JP 11-181600 A relates to a method for checking the electrical contact of connecting pins of an electroplating device. The connecting pins are connected to a conductive film which is provided on the wafer. In order to check the electrical contact of a connecting pin, the electrical resistance of this connecting pin is determined with respect to the other connecting pins.
A conventional method for making contact uniformly over the entire rear side is shown, for example, in U.S. Pat. No. 5,209,833. An electrolytic contact is made with the rear side of the substrate, which contact ensures a very low fluctuation of the contact resistance between the substrate and electrolyte.
The method for making electrolytic rear-side contact over the entire surface is, however, costly in terms of process technology because the electrolyte always requires a wet cell.
It is accordingly an object of the invention to provide a method for electrically contacting a rear side of a semiconductor substrate which overcomes the above-mentioned disadvantages of the heretofore-known methods of this general type and which provides a uniform contact with the semiconductor substrate.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for electrically contacting a rear side of a semiconductor substrate when processing the semiconductor substrate, the method includes the steps of:
providing a semiconductor substrate having a substrate rear side and a substrate front side disposed opposite from the substrate rear side and, if appropriate, removing an insulating layer disposed on the substrate rear side; and
placing the semiconductor substrate with the substrate rear side on a substrate holder such that an electrically conductive contact layer formed of a semiconductor material is disposed between the semiconductor substrate and the substrate holder.
In other words, the object of the invention is achieved by a method for making electrical contact with the rear side of a semiconductor substrate during its processing, wherein a substrate is provided, which has a substrate rear side and a substrate front side located opposite the latter, with an insulating layer on the substrate rear side removed from it, and the substrate is provided with its substrate rear side on a substrate holder, and an electrically conductive layer made of a semiconductor material is provided between the substrate and the substrate holder.
The advantage of the method according to the invention is due to the fact that an electrolytic contact which would require a costly wet cell can be dispensed with. Instead, the electrical connection is made according to the invention through the use of an electrically conductive contact layer which can be used in combination with a substrate holder which is, for example, a metallic or metal-coated vacuum chuck. Here, for example, the conductive contact layer is firstly provided on the chuck and the substrate is subsequently provided on the conductive contact layer.
The electrical contact serves, for example, in electrical or electrochemical processes, to permit a uniform flow of current from the front side of the substrate to the substrate holder.
In one embodiment of the invention, the conductive contact layer is formed as a diffusion barrier for materials of which the substrate holder is composed. Without a diffusion barrier, a vacuum chuck provided with a gold layer could contaminate a semiconductor substrate composed of silicon, the gold atoms forming impurities or defects in the silicon substrate, as a result of which the functions of a field effect transistor can be disrupted.
A further embodiment of the invention provides for a conductive contact layer to be formed from a semiconductor material. The use of a semiconductor material, or preferably the use of the same semiconductor material as the substrate can suitably prevent the contamination of the substrate.
Furthermore, according to another mode of the invention, a conductive contact layer is doped with the same type of charge carrier or dopants as the substrate. Using the same type of charge carrier advantageously avoids a pn-type junction which has a diode blocking effect in a direction of current flow. By using the same type of charge carrier, a low-impedance electrical contact is made possible between the substrate and the conductive layer. In order to achieve a low resistance in the conductive contact layer, the conductive contact layer is provided with a high dopant concentration. Because the substrate is usually rather weakly doped on its rear side, the high doping of the conductive contact layer achieves a homogenous distribution of current on the rear side of the substrate.
According to another mode of the invention, a trench is formed in the conductive contact layer in a surface facing the substrate. The formation of a trench in a surface of the conductive contact layer facing the rear side of the substrate makes it possible to press the rear side of the substrate and thus the substrate against the conductive contact layer by virtue of a vacuum provided in the trench. Pressing the substrate against the conductive contact layer permits a low-impedance contact between the conductive contact layer and the substrate.
According to another mode of the invention, a mesa is formed in the conductive contact layer in a surface facing the substrate. A mesa is, in contrast to a trench, a structure which is surrounded by a trench and which projects beyond its surroundings as an elevation. The mesa is, for example, suitable for an operation in which the rear side of the substrate is pressed against the mesa and a low-impedance electrical contact is thus achieved between the rear side of the substrate and the mesa. This can be achieved through the use of a vacuum which is provided in a trench which surrounds the mesa.
The trenches and mesas which are provided on the surface of the conductive contact layer facing the substrate serve to provide a uniform distribution of the contact pressure of the substrate against the conductive contact layer by virtue of a suitable configuration of trenches and mesas. For example, a checkerboard-like pattern of trenches and/or mesas is suitable here for providing, on the entire rear side of the substrate, a uniform distribution of the vacuum and electrical contact which can be provided by the mesas.
Furthermore, a hole may be formed in the conductive contact layer which hole extends from a surface facing the substrate to a surface, facing the substrate holder, of the conductive contact layer. The configuration of a hole which extends through the conductive contact layer makes it advantageously possible to use the vacuum device of the vacuum chuck for pressing the substrate against the conductive contact layer. This is particularly advantageous because vacuum chucks are already equipped with a vacuum connection and vacuum holes in the surface of the vacuum chuck facing the substrate. This avoids the necessity of a separate vacuum device for the conductive contact layer.
It is also advantageous if a second trench is formed in the conductive contact layer in a surface facing the substrate carrier. As described above, the first trench serves for pressing the substrate against a conductive contact layer through the use of a vacuum. The second trench serves to conduct the vacuum from the vacuum holes in the surface of the substrate carrier through the conductive contact layer.
Furthermore, it is advantageous to form a second mesa in the conductive contact layer in a surface facing the substrate carrier. A pattern which includes the second trench and the second mesa is suitable, for example, for permitting a process in which the conductive contact layer is positioned on the substrate carrier without fine adjustment to the vacuum openings provided by the substrate carrier. The vacuum is conveyed through the conductive contact layer from the substrate carrier to the substrate through the use of the second trench and the hole.
In a further method step there is provision for a lower pressure to be generated in the trench than at the front side of the substrate. Through the use of the pressure difference between the front side of the substrate and the trench, the substrate is pressed against the conductive contact layer and the conductive contact layer is thus pressed against the substrate holder.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for making electrical contact with the rear side of a semiconductor substrate during its processing, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.